1. Field of the Invention
The present invention relates to an Mg-based negative electrode active material, a method of manufacturing the same, a hydrogen storage alloy electrode, and an alkali secondary battery and, more particularly, a powder material suitable for a negative electrode active material as a main material of a secondary battery using an alkali as an electrolyte, a method of manufacturing the same, a negative electrode structure, and a secondary battery.
2. Related Background Art
Recently, CO2 gas contained in the atmosphere is increasing, so the possibility of global warming by the greenhouse effect is pointed out. For example, a thermal electric power plant converts thermal energy obtained by burning fossil fuel into electric energy. Since, however, a large amount of CO2 gas is exhausted by burning, it is difficult to newly build a thermal electric power plant. Therefore, so-called load leveling is proposed as effective use of electric power generated by a power generator such as a thermal electric power plant. This load leveling is to level the load by storing night power as dump power in a secondary battery installed in a general house and using the stored power in daytime during which the power consumption is large.
Also, the development of a high-energy-density secondary battery essential for electric automobiles which do not exhaust air pollutants, e.g., COx, NOx, and hydrocarbons is being expected. Furthermore, for power supplies of portable apparatuses such as a book personal computer, wordprocessor, video camera, and portable telephone, the development of a small-sized, light-weight secondary battery is an urgent necessity.
Under the circumstances, a so-called Ni-hydride battery (to be referred to as an Ni-hydride battery hereinafter) has been put into practical use. This Ni-hydride battery is a high-capacity secondary battery using a hydrogen storage alloy which stores hydrogen at high density as the negative electrode of an alkali secondary battery.
As the hydrogen storage alloy, a misch metal-based alloy represented by Mm(Nixe2x80x94Coxe2x80x94Mnxe2x80x94Al)5, a transition metal-based alloy represented by Tixe2x80x94Zrxe2x80x94Nixe2x80x94Vxe2x80x94Crxe2x80x94Coxe2x80x94Mn, and a magnesium-nickel alloy such as Mg2Ni and MgNi have been studied. Of these alloys, a misch metal-based alloy and a transition metal-based alloy have been put to use in practice as the electrode material.
Unfortunately, the real capacities of both a misch metal-based alloy and a transition metal-based alloy are lower than their theoretical capacities, so further improvements of these alloys are being desired.
Mg is light in weight and has a large hydrogen storage amount (7.6 wt % for MgH2). Therefore, if Mg can be applied to a battery as its negative electrode, a high-capacity battery is expected. However, Mg has a high hydrogen equilibrium dissociation temperature (the dissociation temperature is 287xc2x0 C. when the hydrogen equilibrium dissociation pressure is 0.1 MPa) and hence has low hydrogen storage.releasing ability at room temperature. So, it is difficult to apply Mg to a battery.
As an electrode using a magnesium-nickel alloy, The 37th Battery Symposium in Japan, page 389 (1996) has announced that a high discharge capacity of 750 mAh/g is obtained in the first charge/discharge cycle by an electrode using an amorphous Mg2Ni alloy powder adjusted by mechanical grinding. However, when the charge/discharge cycle is repeated, the discharge capacity rapidly lowers, so no magnesium-nickel alloy electrode having a stable high discharge capacity is obtained.
To solve the above problems, the present inventors have made extensive studies and found a stable alkali secondary battery using a novel Mg-based alloy and having a high capacity and a long cycle life.
That is, it is an object of the present invention to provide an Ni-hydrogen alkali secondary battery which has a high capacity, suppresses the progress of fine powder formation, and hence has a long cycle life, by using an alloy powder containing Ni, Mg, Zn, and Zr and capable of electrochemically storage and releasing hydrogen as a negative electrode active material.
It is another object of the present invention to provide an Mg-based negative electrode active material for use in the abovementioned high-capacity, long-cycle-life secondary battery, a method of manufacturing the same, and a hydrogen storage alloy electrode.
It is still another object of the present invention to provide an Mg-based alloy negative electrode active material used in a hydrogen storage alloy electrode of an alkali secondary battery, comprising an amorphous alloy containing Ni, Mg, Zn, and Zr and capable of electrochemically storage and releasing hydrogen.
The ratio of the Ni weight to the total weight of Mg, Zn, and Zr is 0.2 to 3.0, preferably 0.4 to 2.6.
Letting Mg(x).Zn(y).Zr(z) be the composition of Mg, Zn, and Zr except for Ni, it is desirable that 80 less than x less than 96 wt %, 1 less than y less than 15 wt %, and 0.01 less than z less than 5 wt %, preferably 85 less than x less than 92 wt %, 2 less than y less than 10 wt %, and 0.1 less than z  less than 3 wt % (where x+y+z=100).
It is still another object of the present invention to provide a method of manufacturing an Mg-based alloy negative electrode active material, comprising the steps of placing powders of Ni, Mg, Zn, and Zr in a vessel, and adjusting the powders by mechanically mixing and grinding the powders.
The present invention includes a method in which the powders are mechanically mixed and ground by rotating the vessel and using the centrifugal force of the rotation.
The present invention includes a method in which the powders are adjusted by mechanically mixing and grinding the powders in an ambient selected from the group consisting of an inert gas and an inert gas containing hydrogen gas at a reduced pressure of 0.5 atm or less.
The present invention includes a method which further comprises a heating step of heating the powders at a temperature of 200 to 700xc2x0 C. for 0.5 to 20 hr after the powders are adjusted by the mechanical mixing and grinding, and a cooling step of cooling the powders after the heating step.
It is still another object of the present invention to provide a hydrogen storage alloy electrode of an alkali secondary battery having a negative electrode active material layer, wherein the negative electrode active material layer is formed by using an Mg-based alloy negative electrode active material defined by the present invention.
It is still another object of the present invention to provide an alkali secondary battery comprising a negative electrode made of a hydrogen storage alloy electrode formed by using the abovementioned Mg-based alloy negative electrode active material, a positive electrode containing a metal oxide as a main constituent material, an alkali electrolyte, and a separator.